Q: SIM Card is not supported in activation

Then the phone is not unlocked. Did you request an unlock from your carrier and they granted it? If so, did you restore the phone to factory with iTunes and receive a "Congratulations, your phone is now unlocked" message? If not, then the unlock is not complete. You need to restore the device with iTunes to factory, not from a backup. When you get the "Congratulations" message, you can restore your backup.

Where, exactly, did you get the phone? How was it unlocked?

Check this document for instructions on unlocking an iPhone: How to unlock your iPhone for use with another carrier - Apple Support

Regardless if what your carrier tech support agent told you, the error message you are seeing is telling you that the phone is, in fact, not unlocked.  If you requested the unlock through the carrier it was locked to, and they approved it, did you then erase the device and restore as new in iTunes?  If you did, was there a message in iTunes at the end of the restore as new process that confirmed the device was now unlocked?

Here are the instructions for restoring your iPhone to factory using iTunes. Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Again, if you got an unlock from your carrier, you needed to finish the unlock, and just trying to activate it with the other SIM will not work, nor will a restore from a backup. You have to restore first to factory to complete the unlock process, then you can restore your backup after that.